Gliederung

Objective

In patients with bilateral deafness due to vestibular schwannomas (neurofibromatosis type 2, NF-2), auditory brainstem implants (ABI) are the only possibility to restore some perception of sound. The aim of the present study was to evaluate, if (1) acoustic stimulation of the ABI results in an activation of the auditory cortex and (2) whether this activation correlates to the clinical function.

Methods

Seven patients (aged 31Â±12 years) underwent PET activation studies performed using dedicated full-ring scanners (Siemens, Erlangen, Germany) 8 months (2-37) after unilateral ABI implantation. In each patient dynamic PET scans were acquired after bolus injection of 1.85 GBq H215O – twice during silence and twice during stimulation via headphones with a standardised laboratory speech test. Reconstructed PET frames acquired up to 90-sec p.i. were added and spatially normalised to the first scan obtained during silence. Scans of the same condition were added and a subtraction image (activation – silence) was calculated. If an activation in the auditory cortex could be delineated, the percent of maximum increase in count-density was obtained from ROI-data. The quality of auditory function at the time of the PET investigation was assigned to 4 classes based on consonant recognition and speech tracking test: 1 = very good, 2 = good, 3 = fair and 4 = moderate.

Results

In patients that achieved very good or good auditory function (n=3) activity in the auditory cortex increased 20.7% ipsi- and 22.7% contralateral to the ABI. Two patients with auditory function between fair and good showed only ipsilateral activation of the auditory cortex (12.4%). In one patient with fair and another with moderate auditory function no activation was seen at all (0%). A significant negative correlation was observed between measured activation in the PET study and class of auditory function (r=-0.9, p=0.0042).

Conclusions

The present data suggest that above a particular limit, parallel to an increase in quality of auditory function achieved with an ABI, H215O PET can demonstrate an increase of auditory cortex activation. Therefore, this method is the first tool to measure auditory pathway activation objectively. Hereby, it might be a helpful adjunct to optimise programming of the ABI speech processor.